1. Field of the Invention
This invention relates to the control of the color and the shade of the background and cursor in the video display terminal. More particularly, this invention relates to a novel circuit for controlling the shade and the color in commercially available color cathode ray (CRT) monitors.
2. Description of the Prior Art
Prior art color monitors were either provided with manual brightness and contrast controls or no controls at all. Such controls affect the shade and brightness of the background and cursor. The prior art manual brightness and contrast controls are analog devices which control the voltage amplitude at the cathode of the CRT so as to change the intensity of the color being painted on the CRT. Those skilled in the art of color monitor controls are aware that such manual analog brightness and contrast controls of the type presently employed in commercially available television sets do not provide for digital brightness and contrast controls. A digital to analog conversion circuit is required.
It is also known in the color television art that automatic analog controls are employed in some of the more expensive television sets for automatically controlling the foreground contrast, brightness and tint. These analog devices are extensions of the aforementioned prior art manual controls.
One of the problems which arise in providing a cursor on a color monitor is that it is difficult to detect the cursor. Heretofore, the color of the cursor was made the color of the foreground characters or the color of the background. In some of the latest improved color monitors, the color of the cursor has been alternated between the color of the foreground character and the background color. When the foreground characters are displayed with a cursor where the characters and the cursor have the same color and same intensity, it is very difficult to detect the cursor. To overcome this difficulty, it has been a common practice to provide a cursor with a circuit which causes it to blink. When the cursor is the same color and intensity as the background color, it is very difficult to detect the cursor even when the cursor is provided with a blinking circuit.
It would be desirable to provide in a color video display terminal automatic color and background shade control circuits which have the capability to enhance the contrast of the alpha/numeric data on the screen without obscuring the characters with a full intensity background.